revlog: add revlogio interface
This lets us separate the logic for different on-disk revlog formats from
the main revlog logic.
changeset: 0:9c2cf2b35aa7
user: test
date: Mon Jan 12 13:46:40 1970 +0000
summary: add foo
changeset: 1:3756a9556b89
tag: tip
user: test
date: Mon Jan 12 13:46:41 1970 +0000
summary: change foo
2 revisions:
9c2cf2b35aa7
3756a9556b89